1. Field of the Invention
The present invention relates to an image reading apparatus for reading an image on a sheet material (original), such as a scanner, a copying machine, a printer, or a facsimile apparatus.
2. Related Background Art
In a conventional image reading apparatus provided in a digital copying machine, a printer, a facsimile apparatus or the like, an original treatment apparatus (ADF: Auto Document Feeder), which is an example of an automatic sheet feeding apparatus for automatically feeding an original in the form of a sheet to the image reading apparatus, is provided so as to allow opening and closing.
One of the reading methods for an image reading apparatus equipped with such an original treatment apparatus is a reading method in which the reading device, which is provided below the original treatment apparatus, is moved to stop at a pre-set position by a motor, light is emitted from a lamp unit onto an original that is automatically fed by the original treatment apparatus and transported at a predetermined speed, to perform scanning thereon, and the reading device detects the reflected light to thereby read the image (Hereinafter, this method will be referred to as flow reading).
FIG. 13 shows the construction of the portion in the vicinity of a reading position of a conventional original treatment apparatus adopting flow reading. In the construction shown in FIG. 13, transport roller pairs (402 and 403) are respectively arranged on the upstream side and the downstream side of the reading position α, and, substantially above the reading position α, there are provided a platen glass plate 404 and a guide member 405 spaced apart therefrom by a predetermined gap. A reading device 401 is moved to the reading position α, and the original P is transported past the reading position α at a fixed speed while being regulated by the gap, whereby the reading of the image is effected while restraining fluctuations in speed during the transport of the original. In this case, the reading position α is provided on the upstream side with respect to the position where the original P is in contact with the platen glass plate 404, and the original P is kept somewhat raised from the platen glass plate 404 while the reading is conducted.
Further, FIG. 14 shows a construction in which, instead of the guide member 405, there is provided a platen roller 406 that can be rotated by a driving means (not shown), with the original being transported while pressed against the platen glass plate 404 with a smaller gap therebetween than in the case of the guide member 405. The platen roller 406 is used as a means for preventing the original P from rising when the reading depth is small as in the case of a contact image sensor (CIS) and the like. In this case, the reading position β is a position which is in the vicinity of the platen roller 406 and at which the original P is in contact with the platen glass plate 404.
The conventional original treatment apparatuses adopting flow reading have the following problems In the construction of FIG. 13, reading is effected at a position where the original P is not in contact with the platen glass plate 404, so that foreign substance (airborne foreign substance) generated during original transport, such as toner dust, paper dust, and rubber dust, is scattered and allowed to remain on the platen glass plate 404. When allowed to remain at the image reading position, such airborne foreign substance may be read as a black lined image.
In the construction of FIG. 14, the reading position is set at the position where the original P is in contact with the platen glass plate 404, so that foreign substance (adhering foreign substance) on the original P, such as ink, correction fluid, or paste, is easily allowed to adhere by being rubbed against the platen glass plate 404. Thus, when allowed to adhere to the reading position β, such adhering foreign substance may be read as a black lined image.
Thus, in the conventional flow reading method, it is difficult to prevent generation of a black lined image in the output read image due to airborne foreign substance, adhering foreign substance, etc. As a means for coping with this, a control method is known according to which any foreign substance at the reading position is detected, the reading position being changed according to the detection result (see, for example, Japanese Laid-Open Patent Application No. 2000-196814).
However, the above control, in which the reading position is changed based on the foreign substance detection result, involves the following problem. Due to the physical difference between the first reading position (the reading position prior to the change) and the second reading position (the reading position after the change), there is generated a difference in the quantity of illumination light incident on the reading device after being reflected by the original, resulting in a difference in white level in the original. In some cases, this is due to the influence of back reflection light from a white member on the back side of the original, such as a Mylar or a white roller, or due to rising of the original.
Regarding the above difference in white level due to the difference in reading position, it is necessary to effect correction such that the white level at the first reading position becomes the same as that at the second reading position.